1. Field of the Invention
The present invention relates to an optical fiber cable having a structure in which optical fiber ribbon stack are accommodated in a cylindrical space.
2. Related Art of the Invention
As an method for achieving a multi-core optical fiber cable, optical fiber ribbon stack each having coated optical fibers (hereinafter sometimes simply referred to as"core") aligned and coated so as to be shaped like a tape are used. A multi-core optical fiber cable can be formed easily by stack of the optical fiber ribbon. When the optical fiber ribbons are stacked simply, however, external force is apt to act on the optical fiber cable so that micro-bending loss increases. As a countermeasure, for example, a structure in which a plurality of optical fiber ribbons are accommodated in a metallic tube is employed in an optical fiber cable disclosed in JP-A-8-278432.
It may be, therefore, thought of that the cylindrical space is enlarged and the amount of the cushioning fillers between the optical fiber ribbons and the metallic cylinder is reduced. However, if the amount of the cushioning fillers is too small, the optical fiber ribbon stack hit against the inner wall of the metallic cylinder when bending force acts on the optical cable. As a result, the optical fiber ribbons are broken or bent precipitously, so that transmission loss increases because of leakage of light.
On the other hand, the linear expansion coefficient of the optical fiber ribbon is smaller than that of the cushioning fillers around the optical fiber ribbons. Accordingly, at a low temperature, the outer tube shrinks more greatly than the optical fiber ribbons, so that the optical fiber ribbons are deformed. However, if the amount of the cushioning fillers around the optical fiber ribbons is too large, there occurs another problem that a limitless number of bends with small radii of curvature are generated, so that transmission loss increases because of leakage of light.
The background-art type structure in which only the optical fiber ribbons are directly accommodated in a cylindrical space, however, has a problem that transmission loss is apt to increase because coated optical fibers located in end portions of the optical fiber ribbons suffer side pressure from the inner wall of the metallic cylinder. Accordingly, there arises a problem on design of the optical fiber cable that a countermeasure to widen the cylindrical space, a countermeasure to limit the number of coated optical fibers (hereinafter simply referred to as "cores" in each of optical fiber ribbon stack or the member of optical fiber ribbon stack accommodated in the cylindrical space, etc. are required to prevent the side pressure from acting on the optical fiber ribbon stack.